In the cellular communication art, land mobile radio networks transmit and receive high frequency signals (greater than 800 MHz) via antennas located at land mobile sites. In order to maximize the geographic area for coverage of the signal, the effective radiated power (ERP) must be maximized. The ERP is the product of the power input to the antenna times the gain factor of the antenna; that is, the solid angle direction of the transmission and reception path of the antenna.
It is known in the art that in order to have high ERP while reducing the absolute power into the antenna, the antenna must necessarily have a high gain factor. In order to increase the gain of an antenna, the physical aperture, that is the height and width of the antenna, must increase and the antenna's beam as defined by the solid cone angle, must necessarily occupy fewer steradians. Thus for instance, an antenna might have a vertical beamwidth of 4.degree., while the horizontal beamwidth may be 30.degree.. These beamwidths thus define the antenna's radiating beam solid cone angle. Typically the smaller the beam solid cone angle, the higher the gain of the antenna.
For cellular communication applications, it is generally required, depending upon the location of the land mobile radio site, to cover 360.degree. of azimuth while the vertical beamwidth may only be 4.degree. in order to effectively cover a geographic area. However, in order to cover 360.degree. of azimuth and maintain high gain, it is typically necessary to use twelve antennas with 30.degree. of horizontal beamwidth each. Of course the cost of such antennas and the availability of mounting space for such antennas present significant difficulties. Furthermore, this number of antennas can present wind loading problems at the antenna tower, as well as provide a detrimental visual appearance.
The use of narrow, azimuthal-beam antennas has been quite limited with respect to the land mobile radio industry. One fairly early method of producing multiple antenna patterns out of a common aperture has been employed using a technique called a Butler-matrix feed. Such a matrix consists of a phasing network with N inputs and N outputs, where N can be any integer number greater than one. This phasing network serves to take each of the N inputs and divide the signal amongst the N output ports with each output port having a fixed phase offset with respect to the other output ports. By properly adjusting the phases between adjacent antennas, the output lobe from the antenna can be electrically steered to the left or fight in a controlled fashion. Each of the N inputs creates a different set of phase shifts on the N outputs and therefore results in N distinct "beams" from a common aperture. FIG. 1 illustrates an example of this phase shifting arrangement for eight inputs and eight outputs (N=8). A discussion of the Butler-matrix feed is presented in "Antenna Engineering Handbook", Second Edition, Richard C. Johnsen and Henry Jasick, McGraw-Hill Book Company. pp. 20-56 through 20-60.
Since it is not necessary to have separate antenna apertures to make all of the required antenna beams, the Butler-matrix feed approach greatly reduces the problems associated with the visual appearance of a plurality of antennas, with the concomitant reduction in wind loading, as well as some cost savings with regard to mounting space. One approach for an antenna driven by such a Butler-matrix is shown in FIG. 2, which illustrates four sets of four co-linear arrays of radiating elements, yielding a 4.times.4 panel of radiating elements.